PROJECT SUMMARY The AdoMet Radical Enzyme (ARE) superfamily catalyzes a wide array of chemical transformations that span sulfur-insertion, isomerization, activation of glycyl-radical enzymes, metallo-cofactor biosynthesis, methylations, oxidations and desaturations. All of these reactions are initiated by similar components: AREs are marked by an active-site amino acid signature of CX3CX2C (which binds a unique [Fe4S4] cluster) and structural elements that bind the required co-factor (or co-substrate) S-Adenosyl-methioning (AdoMet). In all cases, the chemistry of AREs is thought to start with the reductive cleavage of AdoMet by the active site cluster, resulting in a 5'-dA radical that is used in the many, many different chemical transformations listed above. AREs are indeed a superfamily with over 105 distinct members, and recent bioinformatics analyses have addressed ~50,000 sequences, helping to categorize their reactivity in many distinct biological roles/pathways involving the synthesis of small molecules, complex natural products, and protein and nucleic modifications. Further complexity within the ARE superfamily can be found in how many family members have yet-another redox cofactor, either one or two additional `auxiliary' [Fe4S4] clusters, or cobalamin. Together, the catalytic and cofactor diversity of the ARE superfamily articulate how very little we understand about the molecular details that guide the reactivity of AREs, we not only do not understand why different AREs do different reactions, there is little data on the redox properties of ARE family members, which is essential to understand how they can achieve the chemistry that they do. We propose to address that knowledge gap. We have have been recently successful in utilizing the crystallographically-characterized ARE BtrN as a model system for examining the redox potentials of the AdoMet-binding active site ([Fe4S4]Ado) and the BtrN auxiliary cluster ([Fe4S4]Aux) using our unique experience in protein film electrochemistry. Here, we propose to (i) use BtrN as a model system to allow for the systematic assessment of what controls the redox potentials and proton-coupled nature of the active site, (ii) expand our work on BtrN to other AREs that contain additional FeS clusters, allowing for the first direct comparison of redox properties between AREs, and (iii) examine the redox chemistry of cobalamin-containing AREs, in order to further develop protein electrochemistry as a tool for redox enzymology in the ARE superfamily.